1. Field of the Invention
The present invention relates to a waveguide photodetector used in receiver and transceiver modules for optical communications systems, and more particularly to such a waveguide photodetector capable of high speed response and having high reliability.
2. Background Art
In photodetectors adapted to receive incident light on their surface (referred to as “surface-receiving photodetectors”), the received signal light enters the light absorbing layer perpendicular to its surface. Therefore, an improvement in the response characteristics of these photodetectors (i.e., a reduction in the carrier transit time) requires a decrease in the thickness of the light absorbing layer, whereas an increase in their photoelectric conversion efficiency (or external quantum efficiency) requires an increase in the thickness of the light absorbing layer. That is, with surface-receiving photodetectors there is a trade-off between the response characteristics and the photoelectric conversion efficiency. On the other hand, in the case of waveguide photodetectors, in which the signal light enters an end face of the light absorbing layer (i.e., enters the layer parallel to its surface), their photoelectric conversion efficiency can be increased by increasing the length of the waveguide (or the light absorbing layer) even if the light absorbing layer has a small thickness, allowing the desired photoelectric conversion efficiency to be established without taking the carrier transit time into account, and vice versa.
For example, the waveguide photodetector disclosed in Japanese Laid-Open Patent Publication No. 2004-146408 achieves high speed response, namely, has a 3 dB cutoff frequency of 40 GHz or more, due to the small thickness of its light absorbing layer (0.5 μm), and still exhibits high photoelectric conversion efficiency (80% or higher).